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Spate of Cyclones in the North Indian Ocean

Spate of Cyclones in the North Indian Ocean

Tropical Cyclone Maha has taken a sharp turn over the Arabian Sea and is now poised to brush India’s west coast on November 7, 2019. By landfall, forecasters expect the storm to have weakened from its extremely severe peak on November 4, which was when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired this natural-color image. At the time, sustained winds measured 185 kilometers (115 miles) per hour—the equivalent of a category 3 storm on the Saffir-Simpson wind scale.

Considered in isolation, there’s nothing particularly unusual about Maha. However, in the context of the season and the basin, it is the latest in a series of strong tropical cyclones in an area that typically doesn’t see many. In fact, the North Indian basin is usually the least active in the Northern Hemisphere.

Activity in the world’s major ocean basins is visible in this map, which shows the historical tracks of storms between 1842 and 2017 as chronicled by NOAA. Brighter areas indicate where a large number of storm tracks have overlapped.

Notice the scarcity of storm tracks in the North Indian basin—particularly west of India—compared to other basins. On average, this North Indian region sees 4.8 storms per year that reach the strength of a “cyclonic storm” (tropical storm) or greater. Of those, just 1.5 reach the strength of “very severe cyclonic storm” (category 1 hurricane) or greater. For comparison, the northwestern Pacific—the busiest basin—sees an average of 26 tropical storms and 16.5 typhoons every year.

Maha is the sixth cyclonic storm of 2019 in the North Indian basin, and the fifth to exceed category 1 strength. It follows Kyarr in October (category 4), Hikaa in September (category 2), Vayu in June (category 2), Fani in May (category 4), and Pabuk in January (tropical storm). Most cyclones in this region tend to form before or after the monsoon season.

It is also notable that four of these cyclones formed west of India over the Arabian Sea. In a typical year, most cyclones form east of India in the vicinity of the Bay of Bengal.

The surge of strong storms this year is likely related to a climate phenomenon known as the Indian Ocean Dipole (IOD). Similar to the way phases of the El Niño-Southern Oscillation shift sea surface temperatures in the Pacific Ocean, the IOD shifts temperatures in the Indian Ocean in ways that can affect seasonal weather patterns.

In a positive phase of the IOD, winds and ocean circulation cause warmer than usual waters in the basin’s west side and cooler than usual waters to the east. This sets up a convection pattern that results in more rain and storms over the Arabian Sea.

The strong IOD is visible in the map above, which shows sea surface temperature anomalies on October 17, 2019. The map does not show absolute temperatures, but how much the surface layer was above or below that day’s average temperature (measured between 2003 and 2014). The warmer than usual (red) water contrasts sharply with colder than usual (blue) water near Indonesia.

According to news reports, the current “positive” phase of the IOD started in June 2019 and strengthened quickly in September due to strong easterly winds. The same report notes that the IOD this year is the strongest it has been in at least 60 years.

NASA Earth Observatory images by Joshua Stevens, using data from the Multiscale Ultrahigh Resolution (MUR) project, storm track data from NOAA, and MODIS data from NASA EOSDIS/LANCE and GIBS/Worldview. Story by Kathryn Hansen.

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